A. Field of the Invention
The present invention relates to a method and device for selecting constellation points representing data transmitted in a data communication system. Although the invention may be applied to any data communication system, it is particularly suited for systems where constellation signal points are selected from non-uniformly spaced, single-dimenisional signal space.
B. Description of the Related Art
A majority of today's public telephone network transmits information digitally as a series of 8-bit bytes or octets at an 8 KHz clock rate. Portions of today's telephone network, however, are not equipped for direct digital transmission. For example, the subscriber loop between the telephone company and the subscriber's home is typically an analog channel. To transmit information over this analog channel, digital information is converted to an analog voltage amplitude known as pulse code modulation ("PCM"). The PCM signal is transmitted to the subscriber over the analog channel, and then translated back to the digital information.
This digital-to-analog ("D/A") and analog-to-digital ("A/D") conversion of octet values to PCM voltage levels are typically performed in accordance with a non-linear quantizing rule. In North America, this conversion rule is known as .mu.-law. A similar non-linear sampling technique known as A-law is used in other geographic areas of the world such as Europe. The non-linear A/D and D/A conversion is generally performed by a codec (coder/decoder) device located at the interface between the digital network and the analog channel.
For high data transmission rates over the analog channel, it is desirable to use a large number of PCM levels to represent the transmitted data. Often, however, only a selected number of PCM levels, known as a constellation of signal points, is used due to the impairments of the transmission channel. For example, on noisy channels it is difficult to distinguish between PCM voltage levels that are spaced close together, resulting in data transmission errors. To distinguish between different PCM levels, the noise level may require an increased spacing between PCM levels. The spacing between selected PCM voltage levels can be increased by using a reduced subset of the available PCM levels. Using a limited number of PCM levels, however, limits the data rates that may be achieved on noisy channels.
In addition, the selection of PCM levels is also constrained by limits on the amount of power that can be transmitted onto the analog channel. The power of the transmitted PCM levels must be below this defined power level. Thus, higher power PCM levels that increase the spacing between PCM levels may not be selected. In this way, the power level limits the selection of constellation points and thus the noise performance and data transmission rate.
Accordingly, it is desirable to select signal constellations having PCM levels spaced at least a minimum distance apart as large as possible to improve noise performance, while staying within the allowable power level.